A Highly Excited Ammonia Cloud at least 300 pc from the Galactic Centre

Abstract

AbstractA cloud with a l.s.r. velocity of ∼ 160 km s-1 and a half-width of ∼ 25 km s-1 was mapped in the (1,1), (2,2) and (4,4) lines of para NH3 and the (3,3) and (6,6) lines of ortho NH3 using the 40″ arc beam of the 100-m Effelsberg telescope. The cloud was extended some 5′ in latitude and contained a central concentration ∼ 1′.5 in size at ℓ = 1°.59; b=0°.01. Line temperatures of the four higher transitions define a rotational temperature of ∼ 120 K, while the (1,1) and (2,2) alone yield the lower value of ∼ 40 K. The high temperatures, well above that of the dust, are too high to be caused by collisions with H2 molecules in the ground state, but might result from collisions with H2 in excited states. On a (ℓ, v) plot the cloud does not coincide with any generally accepted galactic centre feature but it is possibly located near the periphery of the ‘rotating nuclear disk’, where conditions would be favourable for the production of shocks, excited H2 and possibly high NH3 temperatures.